The so-called Mohican roof as the automobile body roof part is formed, for example, by integrating the laminated body 1 shown in FIG. 5A by welding. That is, the laminated body 1 is made up of two thick-walled workpieces 2, 3 made of high tensile strength steel and having substantially the same thickness, and two thin-walled workpieces 4, 5 made of mild steel and having substantially the same thickness stacked together. The two thick-walled workpieces 2, 3 are inner members facing the cabin side, while the two thin-walled workpieces 4, 5 are outer members facing outside the vehicle. In particular, the uppermost thin-walled workpieces 4, 5 are visually recognized by the user.
Welding of the laminated body 1 is performed by spot welding in which the nugget 6 is formed at a plurality of locations. That is, two thick-walled workpieces 2, 3 and two thin-walled workpieces 4, 5 are stacked in this order from the bottom, and the laminated body 1 is formed. Then, the laminated body 1 is sandwiched between the upper tip 7 and the lower tip 8. At this time, the upper tip 7 contacts the uppermost thin-walled workpiece 5 and the lower tip 8 contacts the lowermost thick-walled workpiece 2. The pressing force F1 by the upper tip 7 and the pressing force F2 by the lower tip 8 are set to be equal.
In this state, for example, a welding current flows from the upper tip 7 to the lower tip 8. Since the thick-walled workpieces 2, 3 have larger electric resistance than the thin-walled workpieces 4, 5, heat generated by energization of the thick-walled workpieces 2, 3 is larger than that of the thin-walled workpieces 4, 5. Accordingly, as shown in FIGS. 5A to 5C, the nugget 6 is preferentially grown between the thick-walled workpieces 2, 3 in the laminated body 1. Therefore, as shown in FIG. 5D, there is a concern that the nugget 6 does not grow to the contact surfaces of the thin-walled workpieces 4, 5, and the thin-walled workpieces 4, 5 are not integrated.